1. Field of the Invention
The present invention relates to a pressure transducer system which provides a digital signal indicating a measured pressure.
2. Description of the Prior Art
Systems which provide for pressure measurement utilizing variable frequency pressure transducers have been established in the prior art. Such systems operate by measuring the time necessary to count a predetermined number of cycles of a variable frequency signal and converting this time period to a digital signal which varies linearly with oscillator frequency. The digital signal can then be manipulated by digital computers to provide a digital output signal which varies linearly with pressure.
Digital outputs are provided from the resonant frequency of a stressed element (beam, wire, cylinder, etc.). Varying applied pressure causes a shift in elements resonant frequency which can be converted into engineering units by measuring and then scaling the output frequency through a polynomial equation. These digital transducers eliminate many of the systematic and parasitic errors found in analog transducer systems.
Although the transducers of this type provide adequate performance in most instances they are not user friendly. The user must generally supply additional hardware and software engineering to integrate the transducer into a particular application. In addition to measuring frequency, a multi-term equation is generally required to resolve pressure into usable engineering units.
Another problem with the prior art type of device is that, if a wide operational ambient temperature is expected, the user is responsible for providing a means of temperature stabilization or compensation because most of these standard type secondary transducers do not include thermal compensation. This additional engineering may be prohibitive for the casual or nontechnical user except for very high volume applications.
Prior art systems, aside from providing a need for additional hardware and software engineering to integrate the output of the transducer into a particular useful output measurement, also utilize transducer elements which require conversion from the measurement of a change in an electrical quantity, which is analog, to a digital format. For example, if the change brought about by the pressure measurement affects an electrical quantity such as capacity a further conversion must be made by means of an oscillator, for example, in order to provide a digital output.
An example of the prior art type of pressure transducer and circuitry is disclosed by the U.S. Pat. No. 3,790,910 to McCormack which provides a conditioning circuit for a capacitance type diaphragm measurement of the pressure. In this device the pressure affects a diaphragm which is mechanically coupled to control the value of a variable capacitor which is in turn connected to a transformer and a variable frequency oscillator. The output signals from the variable frequency oscillator are controlled by a counting and correction circuitry including a read only memory which stores constants related to the particular characteristics of the transducer which is used. A counter and an associated register obtain a digital signal indicative of the oscillator frequency. This system of the prior art functions by providing temperature compensated digital output signals by converting the variable frequency oscillator output signal to a rationalized digital signal which increases monotonically with oscillator frequency and has a predetermined value at a reference frequency. There is also included in this system a temperature indicator which provides an analog signal which is converted to a digital signal representative of transducer temperature. This conversion from the analog temperature to a digital temperature is accomplished by a computer which needs to be externally connected to the device of McCormack. The computer performs the calculations on the output digital signal from the frequency oscillator after the signal has been rationalized and further combines the measured digital temperature in order to provide a linearlized temperature compensated digital pressure signal which varies linearly with pressure in accordance with a predetermined mathematical function depending upon the rationalized digital signal and constant which is stored in the read only memory.
The main drawback to the device of the type disclosed in the McCormack patent is the need for a separate computer for each of the transducers in order to solve the equations and to provide the necessary software support to solve the equations. Other disadvantages of this type of prior transducers include the need for a relatively large power supply and the long warm up time as well as a vibration sensitivity. Needless to say because of the need for the computer associated with each device as well as the construction of the complicated circuitry of the device there existed a problem with regard to the expense involved in the construction of each device as well as the support necessary through the computer requirements.